Currently in commercial aircraft so-called air-assisted air conditioning systems are conventionally used to air-condition the aircraft cabin. An aircraft air conditioning system is used to cool the aircraft cabin, which would otherwise become overheated as a result of heat loads, such as for example insolation, the body heat of passengers and waste heat from equipment on board the aircraft. The aircraft air conditioning system moreover feeds sufficient fresh air into the aircraft cabin to ensure that there is a prescribed minimum oxygen content in the aircraft cabin.
Air-assisted aircraft air conditioning systems generally comprise an air conditioning unit, which is supplied with compressed process air from the engines of the aircraft or from a separate compressor. Before being supplied to the air conditioning unit the process air is pre-cooled in a preliminary heat exchanger by transferring heat to cooling air that has likewise been provided by the engines of the aircraft or a separate compressor. The cooling capacity of the preliminary heat exchanger is controlled by a corresponding control of the cooling air mass flow through the preliminary heat exchanger. In the air conditioning unit the process air, as it flows through a heat exchanger unit, is cooled by transferring heat to ambient air flowing through a ram air channel. The ambient air mass flow through the ram air channel and consequently the cooling capacity of the heat exchanger unit of the air conditioning unit are controlled by opening and/or closing corresponding ram air channel flaps.
The structure of an air conditioning unit 100 described for example in DE 10 2010 054 448 may be seen in FIG. 1. The air conditioning unit 100 comprises a process-air feed line 102, through which hot process air generated by a process air source configured in the form of an aircraft engine or a separate compressor is fed at a high pressure to the air conditioning unit 100. In order to control the process air flow through the process-air feed line 102 a control valve 104 is disposed in the process-air feed line 102. The air flowing through the process-air feed line 102 is conveyed through a first heat exchanger 106, during which it is cooled down to ca. 40° C. to 100° C., and then fed to a compressor 108. In the compressor 108 the process air is compressed and at the same time heated. In order to avoid damage to the compressor 108 and/or to components of the air conditioning unit 100 disposed downstream of the compressor 108, the heating of the air as it flows through the compressor 108 has to be limited to temperatures of ca. 220° C. and 260° C. The cooling of the process air, which is to be fed to the compressor 108, by means of the first heat exchanger 106 therefore enables a greater compression of the process air in the compressor 108 and hence the realization of a higher cooling capacity of the air conditioning unit 100.
From the compressor 108 the compressed process air is fed through a line 110 to a second heat exchanger 112, is cooled as it flows through the second heat exchanger 112 and then conveyed through a line 114 to a turbine 124. If desired, the process air prior to feeding into the turbine 124 may be conveyed for further cooling additionally through at least one further heat exchanger disposed upstream of the turbine 124. In the turbine 124 the air is expanded and at the same time cooled down anew. The compressor 108 is disposed with the turbine 124 on a common shaft 125 and is driven by means of the turbine 124.
The first and the second heat exchanger 106, 112 are disposed in a ram air channel 128, through which cold ambient air flows while the aircraft is flying. When the air-craft is operating on the ground, on the other hand, a feed device 130 configured in the form of a blower is used to convey ambient air for cooling the first and second heat exchanger 106, 112 through the ram air channel 128. While the aircraft is flying, the feed device 130 simultaneously continuously rotates when during flying of the aircraft the turbine 124 drives the compressor 108 and hence also sets in rotation the feed device 130 disposed with the compressor/turbine arrangement on a common shaft 125, in which case however the feed device 130 does not feed an ambient air mass flow through the ram air channel 128.
Both during flying and during ground operation of the aircraft the ambient air flow through the ram air channel 128 is controlled by a corresponding positioning of flaps 136, 138 that are disposed in an inlet region and an outlet region of the ram air channel 128. The feed device 130 is disposed with the compressor 108 and the turbine 124 on a common shaft 125, so that the turbine 124 drives not only the compressor 108 but also the feed device 130. A revolutions counter 140 is used to acquire the rotational speed of the shaft 125. An assembly group comprising the turbine 124, the compressor 108 and the feed device 130 is conventionally referred to as an air cycle machine (ACM), wherein the ACM, if need be, may comprise a plurality of turbines, a plurality of compressors and/or a plurality of feed devices.
Further known from the background art is an air conditioning unit 100, which is shown in FIG. 2 and in which the feed device 130 configured in the form of a blower does not interact with the shaft 125 of the compressor/turbine arrangement, i.e. is not disposed jointly with the compressor 108 and the turbine 124 on the shaft 125. In the air conditioning unit 100 represented in FIG. 2 the ACM therefore comprises merely the compressor/turbine arrangement, not however the feed device 130, which is driven via a shaft 142 by a separate motor 144 configured for example in the form of an electric motor. The revolutions counter 140 then determines the rotational speed of the shaft 142 that drives the feed device 130.
During operation of the air conditioning units of prior art operating situations may arise, in which an actual mass flow of the ambient air flowing through the ram air channel and used to cool the heat exchangers disposed in the ram air channel deviates from a set value, i.e. is too high or too low. The reason for such a deviation of the actual mass flow of the ambient air flowing through the ram air channel from a set value may be for example damage of the ram air channel, gaps between individual ram air channel components or leakages in the ram air channel caused by special ram air channel flaps, which are used to prevent excess pressures or excessively high air mass flows in the ram air channel. A deviation of the actual mass flow of the ambient air flowing through the ram air channel from a set value may moreover be caused by mechanical or electrical faults in the operation of the ram air channel flaps or by blockages inside the ram air channel.
A deviation of the actual mass flow of the ambient air flowing through the ram air channel from a set value may be detected in known air conditioning units by pressure measurements by means of corresponding pressure sensors disposed in the ram air channel or indirectly by means of sensors that are not disposed directly in the ram air channel, and may be evaluated by means of a corresponding software logic integrated for example into the control software of the air conditioning system. These methods are however not only comparatively complex but also occasionally inaccurate, with the result that problems associated with a deviation of the actual mass flow of the ambient air flowing the ram air channel from a set value may be identified either not at all or only by corresponding maintenance activity. A routine check of the ambient air mass flow flowing through a ram air channel of an aircraft in the course of regular maintenance intervals is however undesirable on the grounds of cost.
The underlying object of the invention is to provide an easy-to-implement method of determining an air mass flow flowing through a ram air channel, which is in particular integrated into an aircraft air conditioning system. A further underlying object of the invention is to indicate a device suitable for implementing such a method.